To transmit monitoring data from substation premises to a remote monitoring system, a method such as temporarily storing the data, to which a time is attached, and transmitting the data while coordinating an amount of communication data to be transmitted to the remote monitoring system with an amount of stored data is conventionally employed, as described in Patent Document 1. Further, all required data are transmitted.
In recent years, data transmission methods applied to a substation monitoring control system have been set in accordance with international IEC standards such that on substation premises, IEC 61850 is applied to a communication network and IEC 0870-5-103 is applied to a protection control device, while IEC 60870-5-101 or IEC 60870-5-104 is applied to communication between the substation and an upper level control center for the purpose of remote monitoring control.
These IEC standards describe individual contents, respectively, and in a substation monitoring control system, signals from the upper level control center and signals from the substation must be mutually converted by appropriately switching the transmission method.
FIG. 10 is a block diagram showing an example of a substation monitoring control system to which the aforementioned IEC standards are applied. In FIG. 10, 10 denotes a server, 20 denotes a line terminal on the substation premises, and 30 denotes an upper level control center in a remote location.
To simplify the description, terminal numbers such as “1” to “k”, “m”, and “x” (where k, m, and x are integers of two or more, and k<m<x) are allocated to the line terminals 20. Further, in order to identify individual elements, a hyphen and an individual terminal number are added to the reference numeral “20” of the respective blocks indicating the individual line terminals 20.
As shown in FIG. 10, a plurality of line terminals 20-1, . . . , 20-k on the substation premises having the terminal numbers “1” to “k” are connected to the server 10 of the substation monitoring control system via a transmission line (an internal communication network) 41, and IEC 61850 is applied as a signal transmission method between the line terminals 20 having the terminal numbers “1” to “k” and the server 10.
Further, a plurality of line terminals 20-m, . . . , 20-x on the substation premises having the terminal numbers “m” to “x” are connected to the line terminal 20-k having the terminal number “k”, which is connected to the server 10, and IEC 60870-5-103 is applied as a signal transmission method employed on a transmission line 42 between these line terminals. As a result, the terminals 20-m, . . . , 20-x having the terminal numbers “m” to “x” transmit signals to the server 10 via the line terminal 20-k having the terminal number “k”, which serves as a master station.
Furthermore, the server 10 is connected to the upper level control center 30 via a transmission line 43 to which IEC 60870-5-101 or IEC 60870-5-104 is applied.
Among the aforementioned IEC standards, IEC 60870-5-101 and IEC 60870-5-104, which describe transmission methods for remote monitoring control communication, exhibit lower transmission efficiency than IEC 61850, which is a transmission method applied to a communication network on substation premises. Due to the difference in transmission efficiency between these transmission methods and a difference between a signal amount on the substation premises and a signal amount between the substation and the upper level control center, signal transmission for transmitting a signal from the substation premises to the upper level control center can be broadly divided into two types, namely one-to-one signal transmission shown in FIG. 11 and n-to-one signal transmission shown in FIG. 12.
FIG. 11 is a view showing an example of one-to-one signal transmission employed in the substation monitoring control system shown in FIG. 10. In FIG. 11, a signal SIG1 transmitted from the line terminal 20-1 having the terminal number “1” to the server 10 in accordance with IEC 61850 is transmitted as is from the server 10 to the upper level control center 30 as a signal SIG-A in accordance with IEC 60870-5-101 or IEC 60870-5-104. One-to-one signal transmission therefore signifies signal transmission in which a one-to-one correspondence relationship is realized between a substation side signal and a signal bound for the upper level control center.
FIG. 12 is a view showing an example of n-to-one signal transmission employed in the substation monitoring control system shown in FIG. 10. In FIG. 12, signals SIG2 to SIG6 transmitted from the line terminals 20-1, 20-2, 20-k having the terminal numbers “1”, “2”, “k” to the server 10 in accordance with IEC 61850 are aggregated into a signal SIG-B by the server 10 and then transmitted to the upper level control center 30 in accordance with IEC 60870-5-101 or IEC 60870-5-104.
Among these signals, the signals SIG2 to SIG4 are transmitted directly to the server 10 from the line terminals 20-1, 20-2, 20-k. The signal SIG5 and the signal SIG6, on the other hand, are obtained by converting signals SIG7, SIG8, which are transmitted from the line terminals 20-m, 20-x having the terminal numbers “m”, “x” in accordance with IEC 60870-5-103, in the line terminal 20-k.
Hence, n-to-one signal transmission signifies signal transmission in which a plurality of (n) signals from the substation premises are aggregated into a single aggregate signal before being transmitted to the upper level control center such that an n-to-one correspondence relationship is realized between the substation side signals and the signal bound for the upper level control center.
Patent Document 1: Japanese Patent Application Laid-open No. 2002-233081
Patent Document 2: Japanese Patent Application Laid-open No. 5-292572
Incidentally, in the case of the n-to-one signal transmission shown in FIG. 12, international standards do not provide any specific definitions of a correlative relationship between a signal order in which the signals from the substation premises reach the server 10 and a generation time attached to the signals.
As shown in FIG. 13, therefore, no correlative relationship exists between the sequence {T1, T2, T3, T4, T5} of generation times T1 to T5 attached to the signals SIG2 to SIG6 transmitted to the server 10 from the substation premises and the sequence {ST1, ST2, ST3, ST4, ST5} of arrival times ST1 to ST5 at the server 10.
Hence, as shown in FIG. 13, a situation may arise whereby, of the signals SIG2 to SIG6, the signal SIG2 having the latest generation time T5 reaches the server at the earliest arrival time ST1 while the signal SIG5 having the earliest generation time T1 reaches the server at the latest arrival time ST5. During normal aggregation processing in this case, the generation time T5 of the signal SIG2 that arrives first, despite being the latest generation time of the signals SIG2 to SIG6, is attached inappropriately as the generation time of an aggregate signal obtained by aggregating the signals.
As shown in FIGS. 14 and 15, this situation arises due to a time lag generated in accordance with the data transmission method corresponding to the IEC standard. This point will be described below.
FIG. 14 is a view showing a time lag occurring between the occurrence of an event and transmission when a signal is transmitted from the line terminal 20-1 to the server using the IEC 61850 transmission method. As shown in FIG. 14, with the IEC 61850 transmission method, a time lag LAG1 exists between the occurrence of an event and transmission of the signal, and the time lag LAG1 varies according to a processing condition of the transmission source line terminal 20-1 and the condition of the transmission line. As a result, synchronization cannot be achieved between the plurality of line terminals that transmit signals using the IEC 61850 transmission method.
FIG. 15 is a view showing a time lag LAG2 occurring between the occurrence of an event and transmission to the server when a signal is transmitted between line terminals using the IEC 60870-5-103 transmission method and then transmitted from the line terminal serving as the master station to the server using the IEC 61850 transmission method.
As shown in FIG. 15, when the signal is transmitted between line terminals using the IEC 60870-5-103 transmission method, the line terminals 20-m, 20-x serving as slave stations become capable of signal transmission only after being polled by the line terminal 20-k serving as the master station.
Therefore, if an event occurs in the slave station line terminal 20-m following a poll POL1 from the master station line terminal 20-k, for example, as shown in FIG. 15, a signal is transmitted from the line terminal 20-m to the master station line terminal 20-k only when the line terminal 20-m receives a poll POL3 to the line terminal 20-m following a poll POL2 sent to the other slave station line terminal 20-x. In this case, the master station line terminal 20-k transmits the signal to the server 10 using the IEC 61850 transmission method after receiving the signal from the line terminal 20-m.
Hence, when a combination of the IEC 60870-5-103 transmission method and the IEC 61850 transmission method is used to transmit a signal between line terminals and then transmit the signal from a master station line terminal to the server 10, the time lag LAG2 occurs due to a polling interval, the condition of the individual line terminals, and the transmission line condition.
As is evident from FIG. 15, the time lag LAG2 occurring when the IEC 60870-5-103 transmission method and the IEC 61850 transmission method are combined is larger than the time lag LAG1 occurring when transmission is performed using the IEC 61850 transmission method alone by an amount corresponding to the polling interval.
As described above, with a conventional data transmission method of transmitting a plurality of signals from substation premises to an upper level control center using n-to-one signal transmission, a correlative relationship does not exist between the generation times attached to the plurality of signals transmitted to the server 10 from the substation premises and the order in which the signals reach the server 10. Therefore, even if the signal that arrives first has a late generation time, this generation time may be attached as the generation time of the aggregate signal.